Solid-state imaging devices with higher resolution are used in many commercial applications especially camera and also for other light imaging uses. Such imaging devices typically comprise of CCD (charge coupled device) photo detector arrays with associated switching elements, and address (scan) and read out (data) lines. This CCD technology is matured so much that now days millions of pixels and surrounding circuitry can be fabricated using the CMOS (complimentary metal oxide semiconductor) technology. As today's CCD technology is based on silicon (Si)-technology, the detectable spectral ranges of CCD are limited to the wavelengths below 1 μm where Si exhibits absorption. Besides, CCD based imaging technique has also other shortcomings such as high efficiency response combined with high quantum efficiency over broad spectral ranges. This broad spectral detection is required in many applications. One of them is the free space laser communication where shorter (in visible ranges) and near infrared wavelengths is expected to be used. Image sensor array having broad spectral detection capability, disclosed in this invention, is expected to provide those features not available in today's CCD and other imaging (e.g. InGaAs, HgCdTe, or PbS) technologies. With well design of the array, appreciable resolution can also be achieved in image sensor array technology.
Detectors (a.k.a. photodiode or sensor pixel) especially of p-i-n type have been studied extensively over the last decade for its application in optical communication. These photodiodes are for near infrared detection, especially the wavelength vicinity to 1310 and 1550 nm, where today's optical communication is dealt with. Today the photodetector speed as high as 40 Gb/s, as described in the publication by Dutta et. al. in IEEE Journal of Lightwave Technology, vol. 20, pp. 2229-2238 (2002) is achieved. Photodetector having a quantum efficiency as close to 1, as described in the publication by Emsley et. al., in the IEEE J. Selective Topics in Quantum Electronics, vol. 8(4), pp. 948-955 (2002), is also available for optical communication. These photodiodes use InGaAs material as absorption material, and the diode is fabricated on the InP wafer. On the other hand, Si substrate is used for the photodiode for detection of visible radiation. Other materials such as PbS, InAs, InSb, PtSi, PbS, and HgCdTe have been used for detectors for wavelengths greater than 1.65 μm but they generally have to be cooled to low temperature, often have very slow responses or have high dark current. Additionally, HgCdTe is plagued by material growth issues and narrow bandgaps also of InAs and InSb, result in detectors with large dark currents at room temperature.
None of current solution can provide broad spectral detection capability ranges from UV to long or mid infrared wavelengths, necessary to replace multiple sensors by one sensor. It is highly desirable to design the sensor having broader spectral detection ranges and can be fabricated on the single wafer.
For covering multiple spectral ranges, two photodiodes fabricated from Si and InP technology and discretely integrated, can be used. Monolithically, wafer bonding technology to bond Si and InP can be used to fabricated the photodiode covering the wavelengths from visible to near infrared. However, the reliability of wafer bonding over wide range of temperature is still an unsolved issue and a high-speed operation is not feasible with a wafer bonding approach. It is highly desirable to have a monolithic photodetector array (forming image sensor), which could offer high bandwidth (GHz and above) combined with high quantum efficiency over a broad spectral ranges (<300 nm to 3500 nm and also to <300 nm to 5500 nm). For using especially in imaging purpose where CCD or Si based image sensor based device are used, the multicolor photodiode array with the possibility to rapidly and randomly address any pixel is also very much essential.
It is our objective to develop a monolithic photodiode array for broad spectral ranges covering from UV to mid-infrared and extending to the long infrared wavelengths. It is also our intension to develop the image sensor fabrication to the same substrate by transferring the different material system on to the single substrate. Furthermore, it is also our objective to propose the bonding technologies as appropriate for the image sensor proposed and integrated circuit as the single device.
Our innovative approach utilizes surface incident type (either top- or bottom-illuminated type) photodiode structure having single absorption layer and consisting of more than micro-nano-scaled 3-dimensional (3-D) blocks which can provide broad spectral response. Utilizing multiple micro-nano scaled blocks help to increase the absorption spectra more than the material used as the absorption layer. In addition, utilizing the multiple nano-scaled 3-D blocks help to increase the absorption over the wavelength due to the multiple reflections and diffractions inside the 3-D structures. The absorption layers will be designed to achieve the required quantum efficiency and also required speed. The photodiode can be used as the single element and also in array form.
According to this invention, depending on the size and pitch of the 3-D blocks, percentage of light absorption over the wavelengths and also broadening the absorption spectra, the absorption spectra and absorption can be controlled.
According to the current invention, photodiodes having broad spectral ranges (<300 to 10,000 nm and beyond wavelengths), high quantum efficiency (>90%), and high frequency response, can be fabricated using a single wafer. According to this invention, in the case of photodiode array, each array can also be operated independently. The manufacturing thereof is also simpler as compared with the prior art. Some applications include multicolor imaging applications such as for astronomical observation, communication etc.